U.S. patent application number 11/492242 was filed with the patent office on 2007-01-25 for insulated power cable.
Invention is credited to Lisa C. Bates, Richard P. Marek.
Application Number | 20070017692 11/492242 |
Document ID | / |
Family ID | 36254590 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070017692 |
Kind Code |
A1 |
Bates; Lisa C. ; et
al. |
January 25, 2007 |
Insulated power cable
Abstract
This invention relates to an insulated power cable comprising a
multi-strand cable of a plurality of bundles of uninsulated wires,
and electrical insulation sheathing the cable, the electrical
insulation having a thickness of from 0.0625 to 0.5 inches (0.16 to
1.3 centimeters) and comprising a plurality of layers of
spirally-wrapped, creped tape, the tape being comprised of at least
50 percent by weight of an aramid material, the tape having a
density of from 0.1 to 0.5 grams per cubic centimeter prior to
being creped.
Inventors: |
Bates; Lisa C.; (Chester,
VA) ; Marek; Richard P.; (Chesterfield, VA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
36254590 |
Appl. No.: |
11/492242 |
Filed: |
July 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11050504 |
Feb 3, 2005 |
7084349 |
|
|
11492242 |
Jul 25, 2006 |
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Current U.S.
Class: |
174/113R |
Current CPC
Class: |
H01B 3/305 20130101 |
Class at
Publication: |
174/113.00R |
International
Class: |
H01B 11/02 20060101
H01B011/02 |
Claims
1. An insulated power cable, comprising: a) a multi-strand cable of
a plurality of bundles of uninsulated wires, and b) electrical
insulation sheathing the cable, the electrical insulation having a
thickness of from 0.0625 to 0.5 inches (0.16 to 1.3 centimeters)
and containing a plurality of layers of spirally-wrapped, creped
tape, the tape being at least 50 percent by weight of an aramid
material, the tape having a density of from 0.1 to 0.5 grams per
cubic centimeter prior to being creped. wherein when operated while
immersed in oil, a temperature difference of 60 degrees C. can be
maintained between the oil and the cable.
2. The cable of claim 1 wherein the density of the electrical
insulation sheathed on the cable is from 0.2 to 0.6 grams per cubic
centimeter.
3. The cable of claim 2 wherein the density of the electrical
insulation sheathed on the cable is from 0.3 to 0.5 grams per cubic
centimeter.
4. The cable of claim 1 wherein the aramid material is a nonwoven
sheet comprising aramid fibers.
5. The cable of claim 4 wherein the nonwoven sheet is an aramid
paper.
6. The cable of claim 1 wherein the aramid material is a
meta-aramid polymer.
7. The cable of claim 6 wherein the meta-aramid polymer is poly
(metaphenylene isophthalamide).
8. The cable of claim 1 wherein the aramid material is a
para-aramid polymer.
9. The cable of claim 6 wherein the meta-aramid polymer is
poly(paraphenylene terephthalamide).
10. The cable of claim 1 wherein the plurality of uninsulated wires
are present in the form of a plurality of bundles.
11. The cable of claim 1 wherein the multi-strand cable has a size
from 8AWG to 1000 MCM.
12. A cable useful in a transformer comprising the cable of claim
1.
13. (canceled)
Description
[0001] The present application is a continuation of Ser. No.
11/050,504 filed Feb. 3, 2005.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to insulated power cables,
particularly insulated cables commonly used in fluid-filled
electrical transformers.
[0004] 2. Description of Related Art
[0005] Insulated cables and insulated winding wires are both used
in fluid-filled transformers. Insulated winding wire is used to
form the winding of the transformer. This winding wire needs to be
sufficiently stiff in order to withstand mechanical stresses that
occur during operation of the transformer. Insulated cable connects
various components within the transformer such as winding taps to
no-load or on-load tap changers, phase interconnections, and
internal windings to bushing connectors. In contrast to insulated
winding wire, insulated cable needs to be sufficiently flexible to
allow easy maneuverability to the connection points. The cable is
then supported mechanically when additional strength is
required.
[0006] The conductor of the windings in a transformer is typically
composed of a number of winding wires individually insulated to
prevent one wire from coming in contact with another. In many cases
these insulated winding wires are rectangular in cross section to
ensure a dense uniform packing of the transformer windings. In
contrast, the insulated cables used in transformers are normally
made from a plurality of bundles of uninsulated wires and are
generally circular in cross section. Since these cables transmit
electricity at high voltages and high amperages, the key
requirement is that they have sufficient insulation to prevent
dielectric breakdown from one cable to the next, which could be
catastrophic in an oil-filled transformer. Cables in an oil-filled
transformer have traditionally been insulated with spiral-wound,
creped cellulosic paper tapes, and the size and number of cables
used in a transformer were determined by first specifying the
desired maximum temperature difference between the wire cable and
the transformer oil while under load, and then using enough cables
to handle the desired current without exceeding the required
maximum temperature difference. For cellulosic paper tapes, the
maximum temperature difference was generally about 20 degrees
Celsius (Transformer Engineering, Second Ed., published by John
Wiley and Sons, Page 321), because any higher temperature
difference could cause premature aging of the cellulosic insulation
and eventual cable failure.
[0007] However, if the cables could be operated at higher
temperature, that is, if the maximum temperature difference could
be increased to around 60 degrees Celsius, the size of the cables
and/or the number of cables needed for the transformer could be
reduced. Therefore, what is needed is a cable that can withstand a
higher temperature without premature aging of the insulation.
[0008] Research Disclosure RD10833, April 1973 discloses wire
conductors can be wrapped using a "longitudinal-wrapping" technique
wherein a narrow tape of Nomex.RTM. is applied parallel to the
wire, folded around the wire, and sealed. It is preferred to use a
tape that had been creped and then lightly calendered to maintain a
desirable thickness for the insulation.
[0009] Research Disclosure RD10947, May 1973 discloses that in
certain applications where high porosity is desired, such as
insulation for oil-filled transformers, a special low density
paper, e.g. Nomex.RTM. 411 is particularly preferred.
[0010] WO200191135 to Rolling et al. discloses an electrical
apparatus that includes one conductor and an insulation paper
surrounding at least part of the conductor; the insulation paper
includes a wood pulp fiber, a synthetic fiber which can be an
aramid fiber, and a binder material, with the synthetic fiber being
present at between 2 and 25 weight percent. The insulation paper
can be creped and spirally wrapped around the conductor.
BRIEF SUMMARY OF THE INVENTION
[0011] This invention relates to an insulated power cable
comprising a multi-strand cable of a plurality of uninsulated
wires, and electrical insulation sheathing the cable, the
electrical insulation having a thickness of from 0.0625 to 0.5
inches (0.16 to 1.3 centimeters) and comprising a plurality of
layers of spirally-wrapped, creped tapes, the tapes being comprised
of at least 50 percent by weight of an aramid material that has a
density of from 0.1 to 0.5 grams per cubic centimeter prior to
being creped.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a representative of one embodiment of a cable of
this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] This invention relates to an insulated power cable,
particularly insulated cables commonly used in fluid-filled
electrical transformers. The insulated cables of this invention
include a multi-strand cable comprising a plurality of bundles of
uninsulated wires, and electrical insulation sheathing the cable.
One embodiment of the cable of this invention is shown in the
Figure. Insulated cable 1 is shown with a layer of insulation 2
sheathed over a multi-strand cable 3. Multi-strand cable 3 is
comprised of a plurality of uninsulated wires 4 that are preferably
in a plurality of bundles 5. For clarity, the insulated cable shown
in the Figure has an exaggerated amount of open space 6 between the
bundles, however, preferably and generally in practice there is
very limited open space between the bundles.
[0014] The electrical insulation 2 that sheathes the multi-strand
cable 3 has a radial thickness of from 0.0625 to 0.5 inches (0.16
to 1.3 centimeters). An insulation thickness of less than about
0.0625 inches is believed to provide too little amount of
insulation material to provide sufficient dielectric strength. A
thickness of more than about 0.5 inches is believed to provide a
cable that does not permit a reasonable bending radius. The
thickness of the insulation is made up of multiple layers of aramid
material, and the overall density of the sheath of electrical
insulation on the cable is from about 0.2 to 0.6 grams per cubic
centimeter, preferably about 0.3 to 0.5 grams per cubic centimeter.
Since the radial thickness or "build" of the insulation is the
critical parameter, the actual number of layers of materials can
vary, with 10 to 100 layers or more layers being possible. The
layers of aramid material are preferably narrow tapes having a
width of approximately 0.25 to 2 inches. The tapes preferably have
random ridges and grooves, or crepes, across the width of the tape.
The ridges and grooves are imparted into the tape by any available
means, but creping methods that impart a series of random ridges
and grooves are preferred, and micro-creping or dry-creping methods
and equipment such as disclosed in International Patent Application
WO2002/076723 to Walton et al.; U.S. Pat. No. 3,260,778 to Walton;
U.S. Pat. No. 2,624,245 to Cluett; U.S. Pat. No. 3,426,405 to
Walton; and U.S. Pat. No. 4,090,385 to Packard are preferred.
Equipment for micro-creping sheets and tapes can be obtained from
Micrex Corporation of Walpole, Mass. 02081. Such equipment, in
general, presses the tape to be creped against a driven roll that
advances the tape towards a retarding element such as a retarding
blade, the tip of which is held adjacent to the driven roll. The
retarding element causes the tape to be coarsely folded upon itself
by repeated columnar collapse of the tape to form the preferred
ridges and grooves. The tape is preferably mechanically linearly
compacted during the microcreping process about 10 to 200 percent,
preferably 25 to 150 percent, based on the weight increase of the
tape per unit area.
[0015] It is critical that the oil that is used in transformers be
able to penetrate and saturate the insulation around the
multi-strand cable. Therefore, the insulation is applied by
spirally-wrapping the tapes around the cable to form layers that
allow routes for the oil to penetrate and be present between the
layers of the insulation. As used herein, "spirally-wrapped" is
meant to include spiral or helical wrapping of one or more tapes
around the outer circumference of the cable. More importantly, the
aramid material used in the tapes must have a density, prior to
creping, of about 0.1 to 0.5 grams per cubic centimeter, which
provides an insulation having enough porosity to allow the oil to
fully saturate the tape material after it has been wrapped on the
multi-strand cable. Creping of the tapes provides the tapes with
some extensibility so that it can be tightly wrapped around the
cable while at the same time eliminate any stiffness that might be
imparted to the cable from use of a stiff tape. In certain
embodiments of this invention the tapes are made from "formed"
paper that has been made on a wire and lightly compressed but not
substantially densified by the additional application of high heat
and pressure, by for example, a set of heated calender rolls. This
aramid material can be any nonwoven sheet material comprising
aramid fibers that can be slit into tapes, and can be various types
of spunbonded, spunlaced, or paper-like sheets or laminated
structures. In a preferred embodiment, the nonwoven sheet material
is an aramid paper. As employed herein the term paper is employed
in its normal meaning and it can be prepared using conventional
paper-making processes and equipment and processes. The thickness
of the aramid nonwoven sheet or paper (prior to creping) is not
critical but typically ranges from about 0.002 to 0.015 inches.
[0016] The preferred aramid papers used in this invention are
typically made by forming a slurry of aramid fibrous material such
as fibrids and short fibers which is then converted into paper such
as on a Fourdrinier machine or by hand on a handsheet mold
containing a forming screen. Reference may be made to Gross U.S.
Pat. No. 3,756,908 and Hesler et al. U.S. Pat. No. 5,026,456 for
processes of forming aramid fibers into papers.
[0017] As employed herein the term aramid means polyamide wherein
at least 85% of the amide (--CONH--) linkages are attached directly
to two aromatic rings. Additives can be used with the aramid and,
up to as much as 10 percent, by weight, of other polymeric material
can be blended with the aramid or that copolymers can be used
having as much as 10 percent of other diamine substituted for the
diamine of the aramid or as much as 10 percent of other diacid
chloride substituted for the diacid chloride of the aramid. In the
practice of this invention, the aramids most often used are:
[0018] poly(paraphenylene terephthalamide) and poly(metaphenylene
isophthalamide) with poly(metaphenylene isophthalamide) being the
preferred aramid.
[0019] The insulation material is comprised of at least 50 percent
by weight of an aramid material. Other materials that can be used
include celluose, polyamide, polyimide, liquid crystal polymer,
polyethylene naphthalate, polyphenylene sulfide, polybenzoxazole,
polybenzimidazole, polyetherimide, polyethersulfone, wholly
aromatic copolyamides such as those sold under the trademark
Technora.RTM., fluorinated hydrocarbons, or any combination
thereof. Preferably these other materials are in the form of fibers
or particles in the paper. Insulation material having less than
this amount of aramid material is not desired because generally it
cannot withstand greater than 130 degrees Celsius operating
temperature. Preferably the insulation material comprises 75 to
100% aramid materials to take advantage of the high temperature
performance of the aramid polymer.
[0020] The multi-strand cable 3 shown in the Figure that is covered
by the insulation is formed from a plurality of uninsulated wires 4
that are preferably present in the form of a plurality of bundles
5. The multi-strand cable in certain embodiments of this invention
has an overall size of from 8AWG to 1000 MCM, preferably of a size
of 1/0 to 750 MCM. The multi-strand cable preferably meets at least
one of ASTM standards ASTM B172, ASTM B173 or ASTM B8 for stranded
copper conductors. Such multi-strand cables are available from Rea
Magnet Wire Company, Inc., of Osceola, Ark. and Southwire Company
of Carrollton, Ga. Two cables of the present invention were made
from a 500 MCM multi-strand cable having 427 copper wires, each
cable having a nominal diameter of 0.924 inches, which was sheathed
by 15 or 36 layers of creped Type 411 aramid paper tapes. Type 411
aramid paper is an undenisified, 100% percent poly (metaphenylene
isophthalamide) paper having a density of 0.31 grams per cubic
centimeter prior to creping. The 15-layer cable utilized 13 tapes
having a width of 1.25 inches (3.175 centimeters), while the
36-layer cable utilized 32 tapes having a width of 1.3125 inches
(3.334 centimeters). Each layer of the aramid paper tape had a
thickness of 0.00834 inches (0.02 centimeters) prior to creping and
a thickness of 0.0255 inches (0.0648 centimeters) after creping.
The tapes were spirally wrapped around the multi-strand cable and
the final insulative sheathing had a thickness, or build, on the
15-layer multi-strand cable of 0.125 inches (0.32 centimeters) and
a thickness or build on the 36-layer multi-strand cable of 0.25
inches (0.64 centimeters). The cable was immersed in mineral oil,
which fully penetrated the sheathed insulation.
[0021] A key benefit of the cable of this invention is that it can
be operated at a higher temperature in the transformer than prior
art cables. The maximum temperature difference between the oil in
the transformer and the cable can be increased to around 60 degrees
Celsius, thereby reducing the number of cables needed for the
transformer without premature aging of the insulation. For example,
a 50MVA, 12470V transformer utilizing three 350 MCM cables with
0.125 inches build of cellulosic insulation would need to operate
with only two of the same cable size insulated with 0.125 inches
build of creped aramid sheet as described by this invention.
[0022] In one embodiment, the cable of this invention is useful as
a cable in a transformer. Another embodiment of this invention is a
transformer comprising the insulative multi-strand cable as
described herein.
* * * * *